1. Field of the Invention
This invention relates to a laminate suitably used for the production of flexible wiring boards, circuit substrates for TAB and HDD suspensions.
2. Description of Prior Art
Various laminates having a metal layer and a heat resistant polymer layer such as a polyimide resin layer have been proposed for use in flexible wiring boards and HDD suspensions. For example, Japanese Unexamined Patent Publication (Kokai publication) No. JP-A-S60-243120 discloses a substrate for flexible wiring boards having an insulating layer containing a specific polyimide compound and a metal foil. The Japanese Kokai publication does not mention the bonding strength between the insulating layer and the metal foil. In fact, the laminate is apt to cause delamination of a circuit formed by etching the metal foil.
Japanese Kokai publication No. JP-A-H09-174756 discloses a polyimide-metal foil composite film in which a polyimide, obtained by reaction of pyromellitic dianhydride with m-tolidine and a diamino compound selected from 4,4′-diaminodiphenyl ether, 4,4′-diaminodiphenyl thioether, 2,2-bis[4-(4-aminophenoxy)phenyl]propane, 4,4′-diaminodiphenylpropane and 2,2-bis[4-(4-aminophenoxy)phenyl]hexafluoropropane, is directly coated over the metal foil. The laminate of the Japanese Kokai publication has good flatness and dimensional stability. However, no attention is paid concerning the bonding strength between the insulating layer and the metal foil in the invention. In fact, the bonding strength of the known laminate is not satisfactory.
Japanese Kokai publications No. JP-A-H01-244841 and No. JP-A-2002-240193 disclose a laminate having a metal foil, a first polyimide layer and a second polyimide layer. The second polyimide layer has a linear expansion coefficient similar to the metal foil. To improve the adhesion between the second polyimide resin layer and the metal foil, the first polyimide layer having good adhesion property to both the metal foil and the second polyimide layer is interposed therebetween. Because a plurality of polyimide resin layers should be formed, a specific production equipment must be used or repeated coating steps must be employed, so that the production efficiency is reduced. Further, when the polyimide layers are etched with an alkaline etching liquid, satisfactory etching pattern is not easily obtainable because the etching speeds of the polyimide layers are different.